Years ago I had a friend whose Yahoo email account was hacked. It was nothing complicated, someone just worked out her password one way or the other and started using her account to send spam while allowing her to continue using it for her own mail. She had no idea what was going on but complained to me that she was getting occasional emails she didn't understand. The emails were returned spam from the mailer daemon when an incorrect address was used. I logged onto her account, found tons of stuff that didn't belong there, and told her to change her password. After that the mystery mails ceased.

kiwifidget: ...I've been getting email replies to emails I've never sent ... Either non-delivery reports, or automated responses. But every now and then, from an actual person.

Yeah sounds more like spoofing. Your email address could have been harvested from their address book or email conversations. It's very easy to change the "reply to" address to be different from the senders address. And AFAIK, not much you can do about that.

Something else you could do, is check if any of your email addresses were ever compromised. Visit haveibeenpwned.com and check every one of your email addresses. If yes, maybe the source of the breach will help in some way.

Please keep this GZ community vibrant by contributing in a constructive & respectful manner.

perhaps, but your email is now on spammer lists of email addresses , email address they can pretend to be sending from

Also, it could be that your webmail was hacked into. Go into webmail & check recent logins & the send items.Had the same thing with my long unused hotmail a few years back. It was hacked into & emails were sent to webmail contacts (all 2 of them)